Analysis device for analyzing electrical signals from a heart

Abstract: An analysis device (36) for identifying abnormal conditions in a heart (4), retrograde conduction and tachyarrhythmias in particular, is described. The analysis device (36) comprises a differentiating circuit (42) in order to differentiate ECG signals from the heart (4). The differentiated signal is plotted against the ECG signal, and the radius in the resulting curve is calculated in a calculator unit (44). The radius obtained is then compared with a threshold value in a comparator (46), and a sequence for the radius' progression in relation to the threshold value is obtained and compared with previously sequences stored in a sequence analyzer (50).

Description: The present invention relates to an analysis device for analyzing electrical signals from a heart comprising a measurement unit for generating a measurement signal related to the electrical activity of the heart and an evaluation unit for identifying abnormal conditions in the heart on the basis of the measurement signal. The present invention also relates to a heart stimulator for detecting and treating tachyarrhythmias in a heart. In the diagnosis and treatment of patients with various defects in their cardiac function, the ability to make reliable distinctions between normal and abnormal events in the heart is important. The heart's intrinsic signals, i.e. the ECG signals, have long been used for distinguishing between normal and various abnormal conditions. From the document "Phase Plane Plot of Electrograms as a Marker of Ventricular Electrical Instability During Acute Ischemia: Initial Experimental Results and Potential Clinical Applications" (H. S. Karagueuzian et al.: PACE, 1992, Part II, 2188-2193) it is known, that abnormal heart conditions can be identified by analysing the shape of plots which ensues when the electrogram of the heart and the derivative of the electrogram are used as coordinates in a coordinate system. The ability to distinguish normal heart conditions from abnormal conditions is especially important when it comes to tachyarrhythmias. Examination of the heart rate alone is not enough for reliable determination of whether a tachyarrhythmia is present or not. Some hearts manage to operate normally at rates up to about 200 beats a minute, whereas other hearts develop unstable tachyarrhythmias at rates of only 100-120 a minute. So the use of parameters other than the heart rate is necessary for establishing whether a heart is afflicted by unstable tachyarrhythmia. Retrograde conduction from a ventricle to an atrium is another cardiac defect whose identification is important. Retrograde conduct-on impairs the heart's pumping capacity and can trigger PMT's (pacemaker mediated tachycardia), i.e. a tachycardia evoked and sustained by a heart stimulator. The invention aims to achieve an analysis device for reliable identification of abnormal conditions in a heart. The invention further aims to achieve a heart stimulator which can be used for diagnosing and treating tachyarrhythmias. One such analysis device is achieved in accordance with the invention in that the device of the preamble is devised so the device further comprises a differentiating circuit which is connected to the measurement unit in order to form the derivative of the measurement signal, the evaluation unit is connected to the measurement unit and to the differentiating circuit in order to obtain both the measurement signal and the derivative of the measurement signal as input signals and the evaluation continuously determines the radius from an origin to the curve which ensues when the measurement signal and the derivative of the measurement signal are used as coordinates in a coordinate system, and identifies abnormal conditions on the basis of changes in the radius for each heart cycle. The ECG signal, used as the measurement signal in the present invention, does not always display a difference between normal heart conditions and a tachyarrhythmia or some other abnormal cardiac condition which is large enough to permit reliable discrimination of the different conditions. Utilizing the derivative of the ECG signal and plotting the ECG signal against its derivative yields a closed curve for each heart cycle. The radius of the curve designates the differences between different heart conditions far more distinctly than the ECG signal, or its derivative, taken alone. Normal heart function, retrograde conduction and various tachyarrhythmias in particular then display a clear, distinct difference in the curves obtained from the ECG signal and its derivative. This is mainly because the normal state is inherently stable and supplies a curve which is virtually identical for each heart cycle, which is also the case for retrograde conditions and various tachyarrhythmias, i.e. they produce, in principle, the same sequence of changes in the radius of the said curves in each heart cycle. These different conditions can therefore be easily identified and distinguished from each other. Here, it is advantageous if the evaluation unit is devised so it identifies abnormal conditions on the basis of the changes in radius as a function of the time for each heart cycle. This will increase the system's ability to analyze different courses in which the radius changes for various abnormal conditions. A refinement of the analysis device is achieved in accordance with a preferred embodiment of the invention in that the evaluation unit comprises a comparator which compares the radius with a threshold value. Since the change in radius during a heart cycle will vary widely in different conditions, introduction of a threshold value with which the radius is compared is be advantageous. This comparison becomes especially advantageous if the evaluation unit further comprises a means for determining a sequence of time intervals during which the radius is above or below the threshold value respectively and a RAM in which the sequence is stored, and that the evaluation unit identifies abnormal conditions by comparing the most recent identified sequence with previously stored sequences. The identified sequence designates how the radius changes in relation to the threshold value, i.e. how long the radius was above the threshold value and whether it passed the threshold value a plurality of times in one and the same heart cycle. This supplies a "fingerprint" for normal and abnormal conditions in every individual heart. Here, it is advantageous if the threshold value is set at a value related to the maximum radius during the current cardiac cycle, preferably 50% of the maximum radius. Yet another refinement of the analysis is achieved in accordance with a preferred embodiment of the invention in that the evaluation unit comprises at least one additional comparator which compares the radius with at least one additional threshold value. Even in this instance it is especially advantageous if the evaluation unit is also devised to determine a sequence for the period of time during which the radius is above, below and between the threshold values, storing this information in the RAM, and the evaluation unit identifies abnormal conditions by comparing the latest identified sequence with previously stored sequences. This would result in accurate identification of even more conditions in the heart. A heart stimulator for detecting and treating tachyarrhythmias is achieved in accordance with a preferred embodiment of the invention in that a heart stimulator, comprising a pulse generator for generating and emitting stimulation pulses, an electrode system for delivering the stimulation pulses to heart tissue, a detector for detecting tachyarrhythmias and a control device for controlling the pulse generator's generation and emission of stimulation pulses when tachyarrhythmia is detected, is characterized in that the detector comprises an analysis device according to any of the claims 1 - 7. The heart stimulator can naturally also be devised to identify retrograde condition and prevent the development of PMT with an analysis device according to any of the claims 1 - 7. The invention will now be described below in greater detail, referring to the figures in which FIG. 1 shows one embodiment of a heart stimulator with an analysis device according to the invention; FIG. 2 illustrates a normal ECG signal and its derivative plus an ECG signal for a tachyrhythmia and its derivative; FIG. 3 is a diagram showing the curves for a normal heart sequence and for a tachyarrhythmia; FIG. 4 shows the variation in radius of the respective curves in FIG. 3 taken over one heart cycle; FIG. 5 is a bar chart showing the relation of the radii to the threshold values; FIG. 6 shows a second embodiment of the analysis device; FIG. 7 illustrates detection of retrograde conduction from an atrium; and FIG. 8 illustrates detection of retrograde conduction from a ventricle. A heart stimulator 2 is shown in a block diagram in FIG. 1. The heart stimulator 2 is connected to a heart 4 in order to sense heart signals and emit stimulation pulses to the heart 4. A first tip electrode 6 is anchored in an atrium of the heart 4 and connected, via a first electrode conductor 8, to a first pulse generator 10 in the heart stimulator 2. A first ring electrode 12 is connected near the first tip electrode 6 and, via a second electrode conductor 14, to the first pulse generator 10. A stimulation pulse to the atrium can be delivered to heart tissue by the first pulse generator via the first electrode conductor 8 and the first tip electrode 6. The stimulation pulse is then returned, via the first ring electrode 12 and the second electrode conductor 14, to the first pulse generator 10. Alternately, the stimulation pulse can be delivered via the first tip electrode 6 and an indifferent electrode 16 which, in this instance, consists of the enclosure of the heart stimulator 2 but can also consist of a separate electrode located somewhere in the body. The indifferent electrode 16 is connected to the first pulse generator 10 via a third electrode conductor 18 in order to return stimulation pulses from the atrium. A first detector 20 is connected in parallel across the first pulse generator's 10 output terminal in order to sense atrial activity in the heart. In the corresponding manner, a second tip electrode 22 is connected to a ventricle in the heart 4 and, via a fourth electrode conductor 24, to a second pulse generator 26. A second ring electrode 28 is located near the second tip electrode 22 and connected, via a fifth electrode conductor 30, to the second pulse generator 26. Here, delivery of a stimulation pulse to the ventricle can be bipolar via the second tip electrode 22 and the second ring electrode 28 or unipolar via the second tip electrode 22 and the indifferent electrode 16. A second detector 32 is connected in parallel across the second pulse generator's 26 output terminal in order to sense ventricular activity in the heart. The pulse generators 10, 26 and the detectors 20, 32 are controlled by a control device 34 which regulates the stimulation pulses with respect to amplitude, duration and stimulation interval, the sensitivity of the detectors 20, 32 etc. In order to sense and identify abnormal conditions in the heart 4, retrograde conduction and tachyarrhythmias in particular, the heart stimulator 2 is equipped with a special analysis device. In the figure is shown a first embodiment of the analysis device. The analysis device 36 is connected to the first tip electrode 6 via the first electrode conductor 8, the first ring electrode 12 via the second electrode conductor 14, the second tip electrode 22 via the fourth electrode conductor 24, the second ring electrode 28 via the fifth electrode conductor 30 and the indifferent electrode 16 via the third electrode conductor 18. Here, the electrical signals of the heart 4, i.e. the ECG signals, can be detected unipolarly across the first tip electrode 6 and the indifferent electrode 16, across the first ring electrode 12 and the indifferent electrode 16, across the second tip electrode 22 and the indifferent electrode 16 or across the second ring electrode 28 and the indifferent electrode 16. Bipolar detection of the ECG signal is also possible across the first tip electrode 6 and the first ring electrode 12 or across the second tip electrode 22 and the second ring electrode 28. In addition, the electrical signals of the heart 4 can be sensed across any of the electrodes 6, 12 in the atrium and any of the electrodes 22, 28 in the ventricle. In order to achieve this, the analysis device 36 comprises a measurement unit 38 which is capable of selectively receiving signals with any of these combinations and which filters and amplifies input signals in an appropriate manner. The output signal from the measurement unit 38, which output signal is proportional to the measurement signal, is then sent to a buffer 40 and to a differentiating circuit 42. Buffering is performed so the differentiated signal is in phase with the proportional signal when they are sent to a calculator unit 44. The calculator unit 44 calculates a normalized radius for the curve obtained from the two signals used as coordinates in a coordinate system. Calculated radii are sent to a comparator 46 for comparison with a threshold value. When the radius passes the threshold value the first time in each heart cycle, a process starts for determining the time in which the radius is above or below the threshold value during the part of the heart cycle in which an ECG signal is present. The sequence of time intervals obtained here is transferred to a RAM 48. The sequence is then compared in a sequence analyzer 50 with previously stored sequences for normal and abnormal conditions in the heart. When the sequence has been identified, the information is sent to a microprocessor 52 which communicates with the control device 34. If e.g. a tachyarrhythmia is identified, the control device 34 can institute therapeutic treatment with stimulation pulses to terminate the tachyarrhythmia. The microprocessor 52 controls the measurement unit 38 with respect to the measurement signal to be sent to the analysis device 36 and can also control both the comparator 46 for varying the threshold value and the RAM 48 in order to store new sequences not previously found in the heart etc. A physician using a programming unit 56 can, via a telemetry unit 54, communicate with the heart stimulator 2 and thereby obtain information on identified conditions and also re-program the heart stimulator's 2 different functions. In FIG. 2 is shown a normal ECG signal 58 and an abnormal ECG signal 60, in this instance a tachyarrhythmia. As FIG. 2 shows, determining whether a beat was normal or abnormal on the basis of the ECG signal is not easy. Nor can the study of the respective signal's derivative, i.e. the normal ECG signal's derivative 62 and the abnormal ECG signal's derivative 64, reliably determine whether a heart beat was normal or not. However, differences between the signals become apparent when the signals are instead normalized and plotted against each other in a diagram with the proportional signal P on one axis and the differentiated signal D on the other axis. This is illustrated in FIG. 3 in which the normal ECG signal 58 has been plotted against its derivative 62, resulting in the curve 66. The abnormal ECG signal 60 has been plotted against its derivative 64 and given rise to the dashed curve 68. The clearest differences are in the lower part of the P-D diagram in which the abnormal signal curve 68 dips much lower and also has displays a much larger loop around the origin than the normal signal curve 66. When the radius from the origin out to the respective curve is determined during a heart cycle, a simple measure is obtained of the basic, characteristic features of the respective signal. This has been performed in FIG. 4, which illustrates the change in the radius for the normal heart signal 70 and the abnormal heart signal 74 during a heart cycle. In FIG. 4, a first threshold value 72 has also been entered for the normal heart signal 70, said first threshold value 72 corresponding to 50% of the maximum radius during the heart cycle. In the corresponding manner, a second threshold value 76 has been entered for the abnormal heart signal 74, said second threshold values constituting 50% of the maximum radius during the abnormal heart cycle. One simple way of identifying and discriminating the different heart conditions is to measure how long the respective radius distribution is above the respective threshold value during a heart cycle and whether the threshold value is crossed a plurality of times during a heart cycle. In the latter instance, the time in which the radius was on either side of the threshold value is also determined. In FIG. 5 is shown a bar 78 for the normal heart signal 70 and bars 80 for the abnormal heart signal 74 from FIG. 4. The height of the bar 78 corresponds to the time in which the radius was above the first threshold value 72 and corresponds to a normal sequence. The bars 80 for the abnormal heart signal 74 clearly show that the sequence is completely different. When these sequences are stored in the analysis device's 36 memory, each heart cycle can readily be checked as to whether it represents a normal heart cycle or tachyarrhythmia. A second embodiment of the analysis device is shown in FIG. 6. The analysis device 82 can be incorporated into a heart stimulator in the same way as the analysis device 36 in FIG. 1 and can, in particular, be incorporated into the heart stimulator 2 in FIG. 1. So the same reference designations are therefore used for the connections with other components in the heart stimulator. A measurement unit 84 can selectively receive electrical signals via any combination of the first electrode conductor 8, the second electrode conductor 14, the third electrode conductor 18, the fourth electrode conductor 24 and the fifth electrode conductor 30. The measurement signal generated here is sent to a buffer 86 and a differentiating circuit 88. The proportional signal and the differentiated signal are then synchronously sent to a calculator unit 90 for calculation of the radius of the curve derived from the respective signal. The calculated radii are sent to a comparator unit 92 in which the radii are compared to two threshold values. The comparator unit 92 generates different output signals, depending on whether the calculated radius is under both threshold values, above one threshold value, or above both threshold values. The output signal from the comparator unit 92 is sent to an evaluation unit 94 which analyzes the output signal sequence from the comparator unit 92 for each heart cycle and compares it with previously stored sequences. The evaluation unit 94 communicates with the control device 34. In FIG. 7 is compared a radius course for a normal heart cycle 96 and a radius course for retrograde conduction 98. The radius courses are based on signals detected in the atrium of a heart. For both courses, a first threshold value 100 and a second threshold value 102 have been marked. The differences can be clearly seen in the figure. Retrograde conduction 98 produces a more pronounced double peak for the radius course during the heart cycle. The threshold values 100, 102 can be selected so the output signal from the comparator unit 92 accentuates this double peak, as the figure shows. Retrograde conduction can also be identified on the basis of signals detected in a ventricle of the heart. This is illustrated in FIG. 8. A normal radius course 104 and a radius course for retrograde conduction 106 are shown. Both signals are normalized. Retrograde conduction 106 clearly displays a much larger maximum radius. Reliable identification of retrograde conduction 106 is achieved if a first threshold value 108 and a second threshold value 110 are included in a suitable manner. The analysis devices 36, 82 can be combined into a single analysis device which performs all the functions described in the aforesaid embodiments. It can then be devised so it compares the radius to a plurality of threshold values, the time a radius is above, below or between the respective threshold values thereby being established. The analysis devices 36, 82 can be implemented in all forms of heart stimulators and defibrillators.

Claims: Analysis device (36; 82) for analyzing electrical signals from a heart (4) comprising a measurement unit (38; 84) for generating a measurement signal related to the electrical activity of the heart (4) and an evaluation unit (44, 46 48, 50, 52; 88, 90, 92, 94) for identifying abnormal conditions in the heart (4) on the basis of the measurement signal, characterized in that a differentiating circuit (42; 88) is connected to the measurement unit (38; 84) to form the derivative of the measurement signal, the evaluation unit (44, 46 48, 50, 52; 88, 90, 92, 94) is connected to the measurement unit (38; 84) and to the differentiating circuit (42; 88) to receive both the measurement signal and the derivative of the measurement signal as input signals, and the evaluation unit (44, 46 48, 50, 52; 88, 90, 92, 94) continuously determines the radius from an origin to the curve which ensues when the measurement signal and the derivative of the measurement signal are used as coordinates in a coordinate system and identifies abnormal conditions on the basis of changes in the radius for each heart cycle. Analysis device according to claim 1, characterized in that the evaluation unit (44, 46 48, 50, 52; 88, 90, 92, 94) identifies abnormal conditions on the basis of changes in the radius as a function of time for each heart cycle. Analysis device according to claim 1 or 2, characterized in that the evaluation unit (44, 46 48, 50, 52; 88, 90, 92, 94) comprises a comparator (46; 92) which compares the radius with a threshold value. Analysis device according to claim 3, characterized in the evaluation unit (44, 46 48, 50, 52) comprises a means (46) for determining a sequence of time intervals during which the radius is above or below the threshold value respectively and a RAM (48) in which the sequence is stored, and that the evaluation unit (44, 46 48, 50, 52) identifies abnormal conditions by comparing the most recent identified sequence with previously stored sequences. Analysis device according to claim 3 or 4, characterized in that the threshold value is set at a value related to the maximum radius during the current cardiac cycle, preferably 50% of the maximum radius. Analysis device according to any of the claims 2 - 5, characterized in that the evaluation unit comprises at least one additional comparator which compares the radius with at least one additional threshold value. Analysis device according to claim 6, characterized in that the evaluation unit comprises a means for determining a sequence in which the radius is above, below and between the threshold values respectively, said sequence being stored in the RAM, and the evaluation unit identifies abnormal conditions by comparing the latest identified sequence with previously stored sequences. Heart stimulator (2) comprising a pulse generator (10, 26) for generating and emitting stimulation pulses, an electrode system (6, 8; 12, 14; 22, 24; 28, 30) for delivering the stimulation pulses to heart tissue (4), a detector (36) for detecting tachyarrhythmias and a control device (34) for controlling the pulse generator's (10, 26) generation and emission of stimulation pulses when tachyarrhythmia is detected, characterized in that the detector (36) comprises an analysis device according to any of the claims 1 to 7.

Inventors: JAERVERUD KARIN, NOREN KJELL, JAERVERUD, KARIN, NOREN, KJELL, Järverud, Karin
Applicants: PACESETTER AB
Application year: 1994
Main classification: A61N1
IPC codes: A61B5, A61N1